Multifunctional sky camera system for total sky imaging and spectral radiance measurement

ABSTRACT

A multifunctional sky camera system and techniques for the use thereof for total sky imaging and spectral irradiance/radiance measurement are provided. In one aspect, a sky camera system is provided. The sky camera system includes an objective lens having a field of view of greater than about 170 degrees; a spatial light modulator at an image plane of the objective lens, wherein the spatial light modulator is configured to attenuate light from objects in images captured by the objective lens; a semiconductor image sensor; and one or more relay lens configured to project the images from the spatial light modulator to the semiconductor image sensor. Techniques for use of the one or more of the sky camera systems for optical flow based cloud tracking and three-dimensional cloud analysis are also provided.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. application Ser. No.14/031,709 filed on Sep. 19, 2013, now U.S. Pat. No. 9,781,363, which isa continuation of U.S. application Ser. No. 13/873,653 filed on Apr. 30,2013, now U.S. Pat. No. 9,565,377, the disclosures of each of which areincorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to total sky imaging techniques and moreparticularly, to a multifunctional sky camera system and techniques forthe use thereof for total sky imaging as well as spectrally-resolvedirradiance/radiance measurement.

BACKGROUND OF THE INVENTION

Total sky imaging systems are powerful tools for various scientific andindustrial applications such as high resolution cloud tracking andshort-term (less than one hour) solar power forecasting. The contrastrequired by sky imaging (˜1:1,000,000), however, far exceeds thecapability of currently available digital imaging sensors (typically1:1,000 or less dynamic range). A cloudy twilight sky (no direct sun)has a radiance of ˜0.1 milliwatts per square centimeter steradian(mW/cm²sr) while direct sun illumination from a clear sky at noon hasradiance ˜1×10⁵ mW/cm²sr. Imaging twilight sky demands a camera systemwithout significant light attenuation.

On the other hand, the high intensity direct sun light withoutsignificant attenuation impinging on imaging sensors causes pixelsaturation and blooming, i.e., accumulation of photoelectrons exceedsthe designed linear-response and charge-holding capabilities of sensorpixels (saturation), which causes the excessive electrons to overflow toits neighboring pixels (blooming). Blooming causes artifacts in sensoroutput which include large image area around sun being smeared out,streak patterns on image, and erroneous pixel values. In extreme cases,even complete failure of a sensor can result from blooming due tobreakdown of the preamplication stage.

To avoid sensor saturation and blooming, the commercial sky camerasavailable today typically use automated sun-tracking mechanical blockersto prevent direct sun light from entering the camera. The usage ofmechanical sun blockers increases the cost of sky cameras and decreasestheir reliability in harsh field environments. Moreover, information onthe sun and the part of the sky adjacent to the sun (typically around a10 degree field of view), which is most important for solar powerforecast, is lost because of the blocking. Moreover, because the sun isblocked, such sky camera systems can only image the sky around the sunand cannot measure the solar radiation. Thus several additionalradiometric instruments such as pyronameters, spectroradiometers, andpyrheliometers (require precise sun tracking mechanism) are typicallydeployed along with sky cameras to measure the direct and diffusiveirradiance from the sun. This significantly increases the overall systemcomplexity and cost.

Therefore, improved total sky imaging systems that overcome theabove-described drawbacks of conventional technologies would bedesirable.

SUMMARY OF THE INVENTION

The present invention provides a multifunctional sky camera system andtechniques for the use thereof for total sky imaging and spectrallyresolved irradiance/radiance measurement. In one aspect of theinvention, a sky camera system is provided. The sky camera systemincludes an objective lens having a field of view of greater than about170 degrees; a spatial light modulator at an image plane of theobjective lens, wherein the spatial light modulator is configured toattenuate light from objects in images captured by the objective lens; asemiconductor image sensor, and one or more relay lens configured toproject the images from the spatial light modulator to the semiconductorimage sensor.

In another aspect of the invention, a total sky imaging system isprovided. The total sky imaging system includes at least one first skycamera system and at least one second sky camera system, wherein thefirst sky camera system and the second sky camera system each includes(i) an objective lens having a field of view of greater than about 170degrees, (ii) a spatial light modulator at an image plane of theobjective lens, wherein the spatial light modulator is configured toattenuate light from objects in images captured by the objective lens,(iii) a semiconductor image sensor, and (iv) one or more relay lensconfigured to project the images from the spatial light modulator to thesemiconductor image sensor, and wherein the first sky camera system andthe second sky camera system are located at a distance of from about 100meters to about 1,000 meters from one another.

In yet another aspect of the invention, a method for cloud tracking isprovided. The method includes the steps of: (a) obtaining a time-lapsedseries of sky images using a sky camera system including (i) anobjective lens having a field of view of greater than about 170 degrees,(ii) a spatial light modulator at an image plane of the objective lens,wherein the spatial light modulator is configured to attenuate lightfrom objects in images captured by the objective lens, (iii) asemiconductor image sensor, and (iv) one or more relay lens configuredto project the images from the spatial light modulator to thesemiconductor image sensor, and (b) computing a velocity of one or morepixels in the time-lapsed series of sky images using optical flowanalysis, wherein pixels in the time-lapsed series of sky imagescorresponding to cloud regions have a finite velocity and pixels in thetime-lapsed series of sky images corresponding to non-cloud regions havea zero velocity.

In still yet another aspect of the invention, a method forthree-dimensional cloud tracking is provided. The method includes thesteps of: (a) obtaining sky images from each of at least one first skycamera system and at least one second sky camera system, wherein thefirst sky camera system and the second sky camera system each includes(i) an objective lens having a field of view of greater than about 170degrees, (ii) a spatial light modulator at an image plane of theobjective lens, wherein the spatial light modulator is configured toattenuate light from objects in an image captured by the objective lens,(iii) a semiconductor image sensor, and (iv) one or more relay lensconfigured to project the image from the spatial light modulator to thesemiconductor image sensor, and wherein the first sky camera system andthe second sky camera system are located at a distance of from about 100meters to about 1,000 meters from one another, (b) measuring a positionof clouds in the images based on x,y,z axes coordinates of each of thefirst sky camera system and the second sky camera system; (c)determining an x′,y′,z′ orientation of each of the first sky camerasystem and the second sky camera system with respect to zenith and northas references; (d) determining an orientation of the clouds with respectto the zenith and north using the position of the clouds in the imagesmeasured in step (b) and the orientation of each of the first sky camerasystem and the second sky camera system with respect to the zenith andnorth determined in step (c); and (e) calculating a position and avertical height of the clouds in the sky using triangulation based onthe orientation of the clouds with respect to the zenith and north fromstep (d) and three-dimensional coordinates of the first sky camerasystem and the second sky camera system.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an exemplary sky camera system fortotal sky imaging and spectrally resolved irradiance/radiancemeasurement according to an embodiment of the present invention;

FIG. 2 is a diagram illustrating an exemplary methodology for use of skycamera system of FIG. 1 for optical flow based cloud tracking accordingto an embodiment of the present invention;

FIG. 3 is a diagram illustrating an exemplary methodology forthree-dimensional cloud analysis according to an embodiment of thepresent invention;

FIG. 4 is a diagram illustrating the geometry for determining cloudheight triangulation using multiple sky camera installations accordingto an embodiment of the present invention;

FIG. 5 is a diagram illustrating an exemplary methodology for usingsolar position (position of the sun) as a reference point for correctingthe measured orientation of the sky camera systems according to anembodiment of the present invention;

FIG. 6 is a diagram illustrating an alternate embodiment of the presentsky camera system having a wide angle lens according to an embodiment ofthe present invention;

FIG. 7 is a diagram illustrating an exemplary apparatus for performingone or more of the methodologies presented herein according to anembodiment of the present invention;

FIG. 8 is a schematic diagram illustrating integration of a dichromaticbeam splitter in the present sky camera system according to anembodiment of the present invention; and

FIG. 9 is a diagram illustrating use of three Euler angles (α, β, γ) todenote the orientation of the sky camera system with respect to areference coordinates (zenith and north) according to an embodiment ofthe present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Provided herein is a sky camera-based multifunctional system that may beused for sky imaging, radiation measurement, and two- andthree-dimensional cloud analysis. Both the design of the sky camerasystem and analysis methods based on images acquired using such a skycamera system are described in detail below.

The present sky camera system combines an image sensor with spatiallight modulator to achieve the necessary ˜10⁶ dynamic range to cover dimtwilight and direct sun and thus circumvents the need for mechanical sunblocking as in conventional systems (see above). As will be described indetail below, the present camera system is multifunctional as it can beused not only for imaging but also for gathering complete spatially andspectrally resolved sky radiation information in the ultraviolet (UV),visible (Vis), near-infrared (NIR) regime most relevant to solar powerforecasting applications. Further, with the appropriate semiconductorimage sensors (see below), the wavelength of radiation measurement canbe extended into the mid-infrared (MIR) and long-wavelength-infrared(LWIR) regimes.

A single such camera system may carry out the functionality and replacethe combination of conventional total sky imager and radiometricinstruments thus providing significantly cost-savings and reliabilityenhancement over known technologies. Moreover, because the presentcamera system can include sun in the scene, it offers advantages forcloud tracking over conventional sky cameras. First, the immediateregion near the sun (around 10 degree field of view typically blockedwhen using conventional sky cameras with mechanical blockers) becomesavailable for cloud recognition such as using an optical flow basedanalysis as described below. It is notable that the cloud adjacent tothe sun is the most relevant for the purpose of very short range (lessthan 15 minutes) forecasting of solar radiation through the clouds.

Second, the position of the sun offers a built-in reference for theorientation of the sky camera. Note that for sky camera systems mountedon moving/mobile carriers such as a research vessel or aircraft whoseorientation is constantly changing, the sun position provides a meansfor correcting the measurement of the orientation of the system camerasystem. The accurate measurement of the camera orientation is importantnot only for time-lapsed cloud imaging and prediction of cloud motionbut also when multiple cameras are to be used for three-dimensionalcloud tracking in which the base height of the cloud can be obtained bytriangulation. In the following, a detailed description will be providedof (1) the design of the sky camera system, (2) optical flow basedmethod for cloud recognition and cloud motion prediction, and (3)methods of triangulation of cloud height including using the sun as thereference to improve measurement accuracy of the orientation of the skycamera system.

Sky Camera Design:

FIG. 1 is a diagram illustrating an exemplary sky camera system 100according to the present techniques. Sky camera system 100 uses anobjective lens (e.g., fish eye lens 102) which has a field-of-view (FOV)greater than about 170° for total sky coverage (when the sky camerasystem 100 is pointed upright/skyward). The lens may or may not have abuilt-in adjustable aperture (i.e., opening through which lighttravels). The adjustable aperture would serve to control the lightintensity reaching the image sensor (thus the brightness of the images)and the focal depth of the camera system.

A fisheye lens causes unavoidable image distortion close to the edge ofits FOV. For applications requiring imaging a large sky area, such assolar-forecasting for solar farms distributed over a large area, clearimaging of the sky closer to the horizon (far away from the sky camera)with lower image distortion may be desirable. For such applications thecamera system may alternatively use a wide angle lens and the camerasystem may be mounted tilted on a rotational stage. In this case, thecamera captures consecutive images while rotating along the axisperpendicular to earth surface for full sky coverage. This alternativeembodiment is shown illustrated in FIG. 5, described below. As is knownin the art, a wide angle lens has a focal length that is smaller thanthat of a standard lens for a given film plane thus having a widerfield-of-view than standard lens.

At the image plane of the objective lens is a spatial light modulator104, i.e., a device capable of programmable control of light absorptionon its different pixels (spatial locations). In general, the spatiallight modulator 104 serves to attenuate the light from bright objects(e.g., the sun) in the images captured by the objective lens to permit acomplete image of the sky to be obtained (as opposed to using sunblockers which as provided above prevent important image data from beingobtained).

Suitable spatial light modulators include, but are not limited to,liquid crystal modulators (those used in liquid crystal display (LCD)and LCD projectors), quantum well modulators, and micro-blinds basedmodulators. Liquid crystal modulators are described, for example, in C.Falldorf et al., “Liquid Crystal Spatial Light Modulators in OpticalMetrology,” 2010 9^(th) Euro-American Workshop on Information Optics(WIO), pgs. 1-3, July 2010, the contents of which are incorporated byreference herein. Quantum well modulators are described, for example, inJ. Piprek et al., “Analysis of Multi-Quantum Well ElectroabsorptionModulators,” SPIE Proceedings 4646-77, Physics and Simulation ofOptoelectronic Devices X, Photonics West January, 2002, the contents ofwhich are incorporated by reference herein. Micro-blinds basedmodulators are described, for example, in U.S. Pat. No. 7,684,105 issuedto Lamontagne et al., entitled “Microblinds and a Method of FabricationThereof;” the contents of which are incorporated by reference herein.

The spatial light modulator allows, for instance, the high-intensityimage of the sun attenuation to be comparable to the rest of the sky sothat both can be fit into the dynamic range of the image sensor 110 (seebelow). The light absorbed by (or transmitted through) pixels of liquidcrystal modulators are controlled by, e.g., an electrical field appliedto the pixels, which controls the phase and light absorption of theliquid crystal material in the pixel. Quantum well-based spatial lightmodulators exploit the electroabsorption properties of, e.g., multiplequantum wells, i.e., the electron and hole wavefunctions in the quantumwells are modified by electrical field applied (to pixels of themodulator) which changes the light absorption in the quantum wells inthe pixel(s). Micro-blinds-based modulators control light absorption inpixels via electrically switching micrometer-sized blinds between fullyopen and fully closed positions. The degree of light attenuation on thespatial light modulator can be controlled dynamically via feedback fromthe image-sensor 110, i.e., where the pixels of the image-sensor aresaturated (too much incident light), light attenuation is increased atthe corresponding pixels on the spatial light modulator—see FIG. 1 wherethe spatial region containing the image of the sun is attenuated. Wherepixels of the image-sensor have low signal level, attenuation isdecreased at the corresponding pixels on the spatial light modulator.Techniques for controlling light attenuation using a spatial lightmodulator are described, for example, in U.S. Pat. No. 5,917,568 issuedto Johnson et al., entitled Adaptive Attenuating Spatial LightModulator,” the contents of which are incorporated by reference herein.

The image formed on the spatial light modulator 104 (the back imageplane of the fisheye lens 102) is then projected to the image sensor 110via one of more relay lenses 106. Relay lenses may be used singularly(or as a group of lenses) and serve to invert an image and to extend theoptical tube. Relay lenses are commonly used in devices such asmicroscopes and endoscopes. Relay lenses are commercially available, forexample, from Edmund Optics, Inc., Barrington, N.J.

Moreover, to collect spectral (wavelength dependent) information, amechanically switched or electronically tunable spectral filteringcomponent 108 is optionally placed between the objective lens and theimage sensor, between the objective lens and the spatial lightmodulator, between the spatial light modulator and the relay lens, orbetween the relay lens and the image sensor. In embodiments includingspectral filtering, the spectral filter component 108 may include, butis not limited to, mechanically switched filter wheels, electrochromaticglass, liquid crystal, or acousto-optical grating for switching betweenthe desired wavelength windows in the UV-Vis-IR wavelength regime.Electrochromatic glass, or electronically switchable glass, permitsusers to regulate the amount of light that passes through the glass by(electronically) changing the opacity of the glass by applying aregulated charge to the glass. Electrochromatic glass is commerciallyavailable, for example, from SAGE Electrochromics, Farribault, Minn.Acousto-optical grated filters are described, for example, in U.S. Pat.No. 5,946,128 issued to Paek, entitled “Grating Assisted Acousto-OpticTunable Filter and Method,” the contents of which are incorporated byreference herein.

Finally, a semiconductor image sensor(s) 110 is used for imaging.Suitable semiconductor image sensors include, but are not limited to,charge-coupled device (CCD) and complementary metal-oxide semiconductor(CMOS) sensors (including those with anti-blooming features). CCD imagesensors are described, for example, in U.S. Pat. No. 8,164,669 issued toCompton et al., entitled “Charge-Coupled Device Image Sensor withEfficient Binning of Same-Color Pixels,” the contents of which areincorporated by reference herein. CMOS image sensors are described, forexample, in U.S. Patent Application Publication Number 2013/0020662filed by Kao et al., entitled “Novel CMOS Image Sensor Structure,” thecontents of which are incorporated by reference herein. In a CCD imagesensor, pixels capture light and convert it to a digital signal at theedge of the chip. In a CMOS image sensor, the light is converted to adigital signal at the pixel itself.

A special type of CCD image sensor called an anti-blooming CCD isparticularly suitable for such sky imaging purpose. An anti-bloomingsensor (CCD or CMOS) incorporates into its pixels “drain channels” foroverflown electrons. Consequently the threshold of incoming photon flux(light power density) to inducing blooming can be 1,000 times greaterthan a typical CCD or CMOS sensor. The usage of such an anti-bloomingsensor would thus enable the total sky area to be imaged without thepixels for the image area around the sun being saturated due toelectrons overflowing from the neighboring pixel where direct sun lighthits.

Additional spectral coverage for IR light greater than 1,100 nm may beprovided with III-V or II-VI semiconductor based IR image sensors. Thisincludes indium gallium arsenide (InGaAs) array image sensors coveringfrom about 600 nm to about 1,700 nm, indium antimonide (InSb) arrayimage sensors covering from about 1.5 micrometers (μm) to about 5 μm,and mercury cadmium telluride (MCT) array image sensors covering fromabout 5 μm to about 10 μm.

Multiple image sensors can be combined to increase the spectral coveragein which case dichromatic beam-splitters (not shown) may be used todirect light in different wavelength regime to different image sensors.A dichromatic beam-splitter is a filter placed, for example, at 45degrees with respect to the incident light beam and reflects light in aparticular wavelength interval(s) and transmits light in otherwavelength interval(s). The dichromatic beam-splitter may be inserted inthe camera system before or after the relay lens. When inserted beforethe relay lens, multiple relay lenses are used for coupling the imageplane of the objective lens to the corresponding image sensors. See forexample FIG. 8, described below.

The overall dimensions of the sky camera system are determined by thechoice of the objective (e.g., fisheye) lens and the relay lens. Thespatial light modulator is placed at the back focal plane of the lens(i.e., the side of the lens opposite the sky which is being imaged). Theback focal plane of the lens is where parallel beams entering a lensconverge, typically on the order of several to tens of millimeters awayfrom the rear surface of a fisheye lens. The spatial light modulator andthe image sensors are placed at conjugate planes (typically tens tohundreds of millimeters from each other) which the relay lens(es) is/aredesigned to conjugate (relay). In one exemplary embodiment, relay lensesare omitted and the spatial light modulator is placed directly on top ofthe image sensor with the two in physical contact or the distancebetween the two no more than the pixel size of the spatial lightmodulator and the image sensor.

The entire sky camera system 100 is placed in a housing for fieldoperation (i.e., to prevent the sky camera system from being affected bydust or rain) and is controlled by appropriate software to carry out thefunctionality of sky imaging and irradiance/radiance measurement underdifferent light conditions. According to an exemplary embodiment, thesky camera system 100 is controlled by a computer-based apparatus 700,such as that shown in FIG. 7 and described below, to which the skycamera system 100 is remotely connected via a wired or wirelessconnection. Apparatus 700 can also serve to collect and process the dataobtained from sky camera system 100.

Prior to field operation, the response (including spectral dependentresponse) of the camera system, i.e., of the semiconductor image sensor,(signal vs. light radiance) is calibrated indoors using an appropriatecalibration light source of know radiation intensity such as acalibration lamp on an integration sphere. Calibration can be achievedby imaging the calibration source. The response of the camera systemwhile the spatial light modulator is set to transparent is the outputsignal at the pixels of the image sensor(s) divided by the knownradiation intensity of the calibration source. For a description ofcalibration using an integration sphere see, for example, P. D.Hiscocks, “Integrating Sphere for Luminance Calibration,” publishedMarch 2012, the contents of which are incorporated by reference herein.Other applicable calibration light sources include solar simulator oroutdoor solar radiation on a clear day.

For field operation a single such sky camera system 100 may carry outthe functionality of various conventional instruments as illustratedbelow. For imaging the sky or cloud tracking, the pixels on the spatiallight modulator occupied by the sun image are attenuated by a factor offrom about 10³ to about 10⁴, while the rest of the pixels in the spatiallight modulator remain transparent. The signal readout from each pixelof the image sensor is then multiplied digitally by the factor ofattenuation on the spatial light modulator to obtain the true sky image.The sky image can also be obtained in the desired spectral regions byappropriately setting the color filtering elements (the spectral filtercomponent 108).

The direct solar irradiance can be quantified with sky camera system 100by integrating the signal from the pixels of the sun image multiplied bythe attenuation factor at the corresponding pixels of the spatial lightmodulator and the response of the camera system calibrated as describedabove. The integration of information received from pixels is described,for example, in U.S. Pat. No. 7,139,025 issued to Berezin, entitled“Active Pixel Sensor with Mixed Analog and Digital Signal Integration,”the contents of which are incorporated by reference herein.Pyrheliometers are typically used to measure direct solar irradiance.Thus, sky camera system 100 functions to make pyrheliometer-typemeasurements. The diffusive solar irradiance can be quantified with skycamera system 100 by integrating the signal from all pixels on the skyimage excluding the sun multiplied by the attenuation factor at thecorresponding pixels of the spatial light modulator and the response ofthe sky camera system. With sky camera system 100, the total solarirradiance (i.e., direct and diffusive) is given simply by integratingthe signal from all pixels on the sky image multiplied by theattenuation factor at the corresponding pixels of the spatial lightmodulator and the response of the camera system. Pyronameters with andwithout mechanical sun blockers are used to measure diffusive solarirradiance and total solar irradiance. Thus, sky camera system 100functions to make pyranometer-based measurements. Thespectrally-resolved solar total irradiance can be quantified with skycamera system 100 by integrating the signal from all pixels on theentire sky image multiplied by the attenuation factor at thecorresponding pixels of the spatial light modulator and the response ofthe sky camera system when the color filtering element (spectral filtercomponent 108) is set to appropriate spectral intervals.Spectroradiometers are typically used to measure spectrally-resolvedsolar total irradiance. Thus, sky camera system 100 also functions tomake spectroradiometer-based measurements.

Optical Flow Based Cloud Tracking:

An exemplary methodology 200 for use of sky camera system 100 foroptical flow based cloud tracking is now provided by way of reference toFIG. 2. The total sky images obtained with the sky camera system 100 canbe used for cloud recognition and motion prediction as per methodology200. Such prediction of cloud motion together with the radiance, directsolar irradiance, diffusive solar irradiance and/or spectrally-resolvedsolar total irradiance measured in the aforementioned fashion also usingthe sky camera system 100 are the key inputs for short term (less than 1hour) solar power forecast. As known in the art, radiance refers to theradiation power incident on a unit area from a particular direction. Theunit for radiance is watts per square centimeter steradian (W/cm²sr).Irradiance refers to radiation power that passes though a unit area fromall directions in the hemisphere above the area. The unit for irradianceis watts per square centimeter (W/cm²).

Clouds may be identified in images via a threshold method based on thefollowing principle. Light coming from a clear sky region without cloudsis dominated by Rayleigh scattering by gas molecules far smaller thanthe light wavelength. Blue (shorter wavelength) solar light has farstronger Rayleigh scattering than red (longer wavelength) solar light,thus the sky appears blue. Light scattered from a clouded region isdominated by Mie scattering by cloud particles comparable to or largerthan the light wavelength, and as a result the scattered light appearswhite or grey. One can take a ratio of the blue pixel intensity to thered pixel intensity on the sky image and set a threshold. When the ratiois above the threshold (close to blue), one allocates the area to be nocloud, otherwise when the ratio is below the threshold (close to whiteor grey), one allocates the area to be clouded. Such cloud recognition,however, has its limitations wherever there is a presence of largeaerosol (dust) concentration in polluted or desert area. The solar lightscattering on aerosols (of size comparable to cloud particle) make thesky appear white/grey even without clouds. Thus the threshold method forcloud recognition becomes inaccurate.

Advantageously, provided herein is an alternative method of cloudrecognition based on optical flow methods. Optical flow methods are usedherein to compute the motion of objects in a time-lapsed series ofimages at every pixel location in the images. Thus, in step 202, atime-lapsed series of sky images are obtained using, for example, thesky imaging system 100 (of FIG. 1). As will be described in detailbelow, the sky imaging system 100 is particularly useful for cloudoptical flow analysis since the present sky imaging system permitscapturing complete sky images, including the sun. By comparison,conventional sky cameras use sun blockers which exclude image data fromthe region of the image containing the sun which would make optical flowanalysis inaccurate for that (blocked) region.

Time-lapse photography is used to accentuate movement of objects whichto the human eye would appear subtle, such as the movement of cloudsacross the sky. In general, time-lapse photography involves acquiringimages at a frequency (also referred to as a frame rate—i.e., thefrequency at which images are acquired) that is (significantly) lessthan the frequency human eyes are used to view the images. In thisparticular application, the sky camera system 100 may be used to capturea sky image once every from about 1 second to about 5 seconds. Thefrequency at which the images are obtained (to acquire the time-lapsedseries of images) depends in part on the weather conditions, for examplethe amount of wind present and thus how fast the clouds are moving. Forinstance, if there is less wind and the clouds are moving (relatively)more slowly across the sky, then the sky camera system 100 may capturean image every 5 seconds. If on the other hand there is more wind andthe clouds are moving (relatively) more quickly across the sky, then thefrequency (frame rate) might be increased to, e.g., one image persecond. One skilled in the art would be able to configure the frame rateto a given set of weather conditions.

Similarly, the number of images taken in the series can depend on theparticular weather conditions. For instance, one goal of methodology 200is to track cloud cover in order to forecast/predict when the sun mightbe occluded by cloud cover and when it will not. That way the amount offuture solar radiation can be estimated. If the sky is clear and few ifany clouds are present, then a greater number of images need to beacquired in order to gain useful data about the cloud movement relativeto the sun's position. Conversely, if there are clouds in the immediatearea of the sun, then a few images might yield enough information foranalysis. In general however, for typical weather conditions, thetime-lapsed series of images will include from about 2 images to about50 images acquired, for example, using the sky camera system 100. Again,one skilled in the art would be able to determine the number of imagesneeded for a given set of weather conditions to determine cloudmovement.

It is notable that in order to have a constant frame of reference (forimage location), it is preferable for the sky camera system 100 whentaking the time-lapsed series of images to remain at a fixed location,especially when a single sky camera system is being used. However, aswill be described in detail below, embodiments are provided herein wheremultiple sky camera systems are employed and used for three-dimensionalcloud tracking.

Details of the optical flow analysis techniques used herein are nowprovided. In the simple case for time-lapsed two-dimensional images, apixel of intensity I(x, y, t) at location (x, y) (i.e., a pixellocation) and time t is assumed to move by Δx, Δy at time t+Δt and thusthe following equation holds:I(x,y,t)=I(x+Δx,y+Δy,t+Δt).  (1)Assuming that the motion between an image and the immediate next imagecaptured is small, applying a 1^(st) order Taylor expansion to I(x+Δx,y+Δy, t+Δt) yields,

$\begin{matrix}{{I( {{x + {\Delta\; x}},{y + {\Delta\; y}},{t + {\Delta\; t}}} )} = {{I( {x,y,t} )} + {\frac{\partial I}{\partial x}\frac{\Delta\; x}{\Delta\; t}} + {\frac{\partial I}{\partial y}\frac{\Delta\; y}{\Delta\; t}} + {\frac{\partial I}{\partial t}.}}} & (2)\end{matrix}$Combining Equations (1) and (2) it follows that:

$\begin{matrix}{{{{\frac{\partial I}{\partial x}V_{x}} + {\frac{\partial I}{\partial y}V_{y}} + \frac{\partial I}{\partial t}} = 0},} & (3)\end{matrix}$wherein

$V_{x} = {{\frac{\Delta\; x}{\Delta\; t}\mspace{14mu}{and}\mspace{14mu} V_{y}} = \frac{\Delta\; y}{\Delta\; t}}$is the x and y velocity of the object at location (x, y). Equation (3)combined with further constraints allows Vx, Vy to be solved. By way ofexample only, the Lucas-Kanade implement of optical flow, furtherconstraints are introduced by assuming (V_(x), V_(y)) are constant inlocal pixel neighborhood. See, for example, B. D Lucas and T. Kanade,“An Iterative Image Registration Technique with an Application to StereoVision,” Proceedings of Imaging Understanding Workshop, pp. 121-130(April 1981), the entire contents of which are incorporated by referenceherein. Another implementation by Horn and Schunck introduces globalsmoothness of the velocity field (V_(x), V_(y)) as the furtherconstraint. See, for example, B. K. P Horn et al., “Determining OpticalFlow,” Artificial Intelligence, 17, 185-203 (August 1981), the entirecontents of which are incorporated by reference herein. Other applicableconstraints are contained in a comprehensive review of optical flowmethods provided by J. L. Barron et al., “Performance of Optical FlowTechniques,” International Journal of Computer Vision, volume 12, page43-77 (February 1994), the entire contents of which are incorporated byreference herein can be used to solve the velocity field (V_(x), V_(y))for every location on the image.

Thus, in step 204, optical flow analysis (see Equations 1-3 above) isused to compute the velocity of pixels in the time-lapsed series ofimages (from step 202). The pixels corresponding to a cloud (and thushaving a certain intensity I) will have a certain finite velocity in thetime-lapsed series of images since the clouds are moving. On the otherhand, the pixels corresponding to the non-cloud sky regions (e.g.,regions of blue sky) (and thus having a different intensity) will have azero velocity in the time-lapsed series of images since those regions ofnon-cloud sky are not moving. By zero velocity we refer to no velocityor any small velocity value within the uncertainty caused by the noisein the images. Such uncertainty can be quantified through analyzing aseries of sample images of clear sky (visually identified to be cloudfree) using the optical flow method.

Thus, the derived velocity field of the sky images can serve twopurposes. First, as per step 206, it allows the discrimination of thecloud vs. non-cloud sky region. Namely, as highlighted above, a piece ofcloud will have the same finite velocity (due to wind) of motion whilethe non-cloud region have a zero velocity. Thus, the pixelscorresponding to cloud regions will have a positive, finite velocitycomputed in step 204, while those pixels corresponding to the non-cloudregions will have a zero velocity. From the pixel locations, the cloudand non-cloud regions in the images can be distinguished. The positionof a cloud can be obtained, for example, by finding the center of massof a group of consecutive pixels of similar finite velocity. Theboundary of a cloud can be obtained, for example, by finding theboundary between a group of consecutive pixels of similar finitevelocity and surrounding pixels. Second, as per step 208, the velocityfield enables short range (less than one hour) forecasting via assumingconstant cloud velocity and linear extrapolation, i.e., for a cloud atlocation (x,y) at time t will be forecasted to be at location(x+V_(x)Δt, y+V_(y)Δt). Thus, given a location of a pixel in thetime-lapsed series of sky images (corresponding to a cloud) and thecalculated velocity of the pixel, linear extrapolation can be used topredict where the pixel/cloud will be at a future point in time.

Unlike the conventional threshold based methodology, the optical flowbased process for cloud recognition and forecast provided herein willwork in spite of a hazy aerosol sky background. As highlighted above,the design of the sky camera system 100 enables cloud close to the sunregion to be recognized and forecasted, which is an advantage over aconventional system based on mechanical blocking.

Multiple Cameras for Three-Dimensional Cloud Tracking:

When a single sky camera is available, the two-dimensional cloudinformation can be extracted using the aforementioned optical flowprocess. Three-dimensional cloud analysis (including cloud height) canbe derived via deploying at least two cameras and triangulation usingsolar, i.e., the sun's position as a convenient reference point asdetailed below.

FIG. 3 is a diagram illustrating an exemplary methodology 300 forthree-dimensional cloud analysis. In step 302, sky images are acquiredat time t using multiple installations of the sky camera systems 100.According to an exemplary embodiment, at least two different sky camerasystems 100 are employed at different locations, the installationsreferred to herein arbitrarily as sky camera system 100A and sky camerasystem 100B. By way of example only, sky camera system 100A and skycamera system 100B are located (e.g., based on three-dimensional GPScoordinates—see below) at a distance of from about 100 meters to about1,000 meters, and ranges therebetween from one another. The distancespecified here refers to inter-camera distance in three-dimensionalspace. When possible, a large distance is preferred to improvetriangulation accuracy (see below). A schematic diagram illustratingmultiple sky camera systems 100A, 100B, etc. is provided in FIG. 4,described below. It is notable that the three-dimensional coordinatesobtained, e.g., using GPS, for the sky camera systems can be used todetermine the displacement from one sky camera system to the other. Thedisplacement is the vector from the first sky camera system to theother. Thus, if sky camera system 100A is at (x1, y1, z1) and sky camerasystem 100B is at (x2, y2, z2), then the displacement is the vector(x2−x1, y2−y1, z2−z1).

For any given fisheye lens, the relationship between the angle of anobject with respect to the optical axis of the lens Θ and the projecteddistance between the image of the object and the optical axis r on theimage plane is known and called the mapping function of the fisheye lensr=f(Θ). In step 304, the mapping function allows the azimuth and zenithangles (Φ, Θ) of any object (such as the sun or a cloud, see FIG. 1)with respect to the camera system to be measured from the position ofthe image of the object on the image plane of the lens (x,y). Thus, instep 304, the solar or cloud position (Φ, Θ) is measured from the images(acquired in step 302) based on the x,y,z axes coordinates of each skycamera system 100A and 100B (see FIG. 1).

To enable triangulation of three-dimensional cloud positions, one mustalso know the orientation of each of the sky camera systems with respectto a standard reference (for example defined by the zenith, opposite tothe direction of gravity and north). See step 306 of methodology 300.The orientation of a rigid body such as a camera with respect toreference coordinates (such as zenith and north) can be represented bythe notation of three Euler angles (α, β, γ) as illustrated in FIG. 9 inwhich xyz represent the axes of the sky camera system (refer to FIG. 1),x′ y′ z′ reference the fixed reference coordinates defined by zenith(z′) and north (x′). The orientation of the sky camera systems can bedetermined in a couple of different ways.

In one exemplary embodiment, the orientations of the sky camera systemswith respect to zenith and north are measured using instrumentation suchas a combination of accelerometer and/or electronic inclinometer and/orgyroscopes and/or magnetometer based electronic compasses as described,for example, U.S. Patent Application Publication Number 2010/0250177 A1,filed by Van Acht et al., entitled “Orientation Measurement of anObject,” and U.S. Patent Application Publication Number 2012/0075109 A1,filed by Wang et al., entitled “Multi Sensor Position and OrientationSystem,” the entire contents of each of which are incorporated byreference herein. In another exemplary embodiment, the orientations ofthe sky camera systems are measured with instruments and then correctedusing solar position as a reference point. This alternative exemplaryembodiment is described in conjunction with the description of FIG. 5,below.

Knowing the azimuth (Φ) and zenith (θ) angles of cloud or sun in the skycamera system coordinate xyz (as defined in FIG. 1) and the orientationof the sky camera system with respect to the reference coordinatesx′y′z′ defined by zenith and north (FIG. 9), the azimuth (Φ₁) and zenith(θ₁) angle of cloud or sun with respect to the x′y′z′ can be calculatedin step 308 using the following equation (refer to FIG. 4).

$\begin{matrix}{\begin{pmatrix}{\sin\;\theta_{1}\cos\;\Phi_{1}} \\{\sin\;\theta_{1}\sin\;\Phi_{1}} \\{\cos\;\theta_{1}}\end{pmatrix} = {\begin{pmatrix}{\cos\;\alpha} & {\sin\;\alpha} & 0 \\{{- \sin}\;\alpha} & {\cos\;\alpha} & 0 \\0 & 0 & 1\end{pmatrix}^{- 1}\begin{pmatrix}1 & 0 & 0 \\0 & {\cos\;\beta} & {\sin\;\beta} \\0 & {{- \sin}\;\beta} & {\cos\;\beta}\end{pmatrix}^{- 1}\begin{pmatrix}{\cos\;\gamma} & {\sin\;\gamma} & 0 \\{{- \sin}\;\gamma} & {\cos\;\gamma} & 0 \\0 & 0 & 1\end{pmatrix}^{- 1}\begin{pmatrix}{\sin\;\theta\;\cos\;\Phi} \\{\sin\;\theta\;\sin\;\Phi} \\{\cos\;\theta}\end{pmatrix}}} & (4)\end{matrix}$

To enable three-dimensional cloud analysis, as highlighted above, atleast two sky camera systems 100A and 100B are deployed with aseparation of from about 100 meters to about 1,000 meters and rangestherebetween from one another. Thus the orientations of clouds withrespect to the zenith and north at the location of the first and secondsky cameras (Θ₁, Φ₁) and (Θ₂, Φ₂) as illustrated in FIG. 4 aredetermined using the aforementioned methods.

Finally in step 310 (Θ₁, Φ₁) and (Θ₂, Φ₂) and the displacement betweenthe two sky camera systems (see above) can be used to derive thevertical height of the clouds via triangulation. See FIG. 4, and usingequation (5) below. The term “displacement,” as used herein refers to athree-dimensional relation between the two camera systems, i.e., basedon the three-dimensional coordinates of the sky camera systems obtainedusing, e.g., GPS (see above). In Equation (5) (x₁, y₁, z₁) and (x₂, y₂,z₂) are the spatial coordinates of the first and second sky camera and(x_(c), y_(c), z_(c)) are the calculated coordinates of the cloud. Thus,in step 310, a vertical height of the clouds in the images can bedetermined using triangulation based on the distance the sky camerasystems 100A, 100B are from one another.

$\begin{matrix}\{ \begin{matrix}{x_{c} = {x_{1} + {\cos\;\Phi_{1}\frac{{( {x_{2} - x_{1}} )\sin\;\Phi_{2}} - {( {y_{2} - y_{1}} )\cos\;\Phi_{2}}}{{\cos\;\Phi_{1}\sin\;\Phi_{2}} - {\sin\;\Phi_{1}\cos\;\Phi_{2}}}}}} \\{y_{c} = {y_{1} + {\sin\;\Phi_{1}\frac{{( {x_{2} - x_{1}} )\sin\;\Phi_{2}} - {( {y_{2} - y_{1}} )\cos\;\Phi_{2}}}{{\cos\;\Phi_{1}\sin\;\Phi_{2}} - {\sin\;\Phi_{1}\cos\;\Phi_{2}}}}}} \\{z_{c} = {z_{1} + {\cot\;\theta_{1}\frac{{( {x_{2} - x_{1}} )\sin\;\Phi_{2}} - {( {y_{2} - y_{1}} )\cos\;\Phi_{2}}}{{\cos\;\Phi_{1}\sin\;\Phi_{2}} - {\sin\;\Phi_{1}\cos\;\Phi_{2}}}}}}\end{matrix}  & (5)\end{matrix}$

FIG. 4 is a diagram illustrating the geometry for determining cloudheight triangulation achievable using multiple sky camera installations.In FIG. 4, sky camera system 100A and sky camera system 100B are labeled“Camera 1” and “Camera 2,” respectively. The zenith with respect to eachof the sky camera systems is labeled, as is north.

To the extent that the accuracy of the sky camera system orientationmeasurement directly affects the triangular accuracy for cloud lateralposition and height measurement, it may be desirable to use the positionof the sun as an independent calibration of the orientation. Seemethodology 500 of FIG. 5. In this exemplary embodiment, the solarposition (position of the sun) serves as a reference point forcorrecting the measured orientation of the sky camera systems.

The accurate solar position (i.e., position of the sun) at any given skycamera location (latitude, location, and elevation—obtained using anyGPS device) and local time can be calculated. The solar position isdefined using zenith angle (with respect to the zenith, defined asdirect opposite to the gravitational force at that location and azimuthangle (with respect to north). For such calculations one may apply theprocess provided by Reda, I., Andreas, A., (2003) “Solar PositionAlgorithm for Solar Radiation Applications, National Renewable EnergyLaboratory (NREL) Technical report NREL/TP-560-34302, Revised January2008, the entire contents of which are incorporated by reference herein.Thus in step 502, the solar position is calculated based on the location(e.g., latitude, location, and elevation) of each of the sky camerasystems 100A, 100B with respect to the true zenith and true north.

The calculated solar zenith and azimuth angle, (Φ₁, θ₁), with respect tothe zenith and north vs. the measured zenith and azimuth angle, (Φ, θ),with respect to the orientation of the sky camera—see step 304 of FIG.3, described above thus enables correction of measurement error of theorientation of the sky camera represented by, for instance, the threeEuler angles (α, β, γ) shown in FIG. 9. This can be done again usingEquation (4), above. As an example, knowing (Φ₁, θ₁) and (Φ, θ),Equation (4) provides two sets of constraints for (α, β, γ) so that withthe knowledge of one angle out of the three, the other two angles can besolved. The tilt angle of the camera optical axis (β) can be accuratelymeasured via an inclinometer (with error as small as 0.01 degrees) andused to solve accurately α and γ. This can serve as a correction for thedirect measurement of α and γ using for example magnetometer basedelectronic compasses which are far less accurate. Such correction of theorientation of the sky camera system is done when land-based (fixed) skycamera systems are first installed. For mobile sky camera systems (sayship-borne systems) the calculation is done continuously to update thesky camera orientation when the sun is available. This feature isparticularly important for mobile sky-camera systems as the orientationof the camera can be continuously changing with ship pitching androlling. Thus, in step 504, the measured orientation of each of the skycamera systems 100A, 100B with respect to the zenith and north iscorrected using the solar position calculated in step 502 as referencepoints.

As provided above, in an alternate embodiment the present sky camerasystem may use a wide angle lens and the sky camera system may bemounted tilted on a rotational stage. Such an exemplary embodiment isshown illustrated in FIG. 6. As shown in FIG. 6, the sky camera systemcan be used to capture consecutive images while the rotating stage turnsthe sky imaging system along the axis perpendicular to the earth'ssurface. Accordingly, full sky coverage can be achieved in a mannersimilar to a fish eye lens.

Turning now to FIG. 7, a block diagram is shown of an apparatus 700 forimplementing one or more of the methodologies presented herein. By wayof example only, apparatus 700 can be configured to implement one ormore of the steps of methodology 200 of FIG. 2 for optical flow basedcloud tracking and/or one or more of the steps of methodology 300 ofFIG. 3 for three-dimensional cloud analysis. For instance, as providedabove, a computer-based apparatus (such as apparatus 700) may beconfigured to control and/or receive data from the present sky camerasystem(s) and to analyze the data.

Apparatus 700 includes a computer system 710 and removable media 750.Computer system 710 includes a processor device 720, a network interface725, a memory 730, a media interface 735 and an optional display 740.Network interface 725 allows computer system 710 to connect to anetwork, while media interface 735 allows computer system 710 tointeract with media, such as a hard drive or removable media 750.

As is known in the art, the methods and apparatus discussed herein maybe distributed as an article of manufacture that itself comprises amachine-readable medium containing one or more programs which whenexecuted implement embodiments of the present invention. For instance,when apparatus 700 is configured to implement one or more of the stepsof methodology 200 the machine-readable medium may contain a programconfigured to (a) obtain a time-lapsed series of sky images using a skycamera system including (i) an objective lens having a field of view ofgreater than about 170 degrees, (ii) a spatial light modulator at animage plane of the objective lens, wherein the spatial light modulatoris configured to attenuate light from objects in images captured by theobjective lens, (iii) a semiconductor image sensor, and (iv) one or morerelay lens configured to project the images from the spatial lightmodulator to the semiconductor image sensor, and (b) compute a velocityof one or more pixels in the time-lapsed series of sky images usingoptical flow analysis, wherein pixels in the time-lapsed series of skyimages corresponding to cloud regions have a finite velocity and pixelsin the time-lapsed series of sky images corresponding to non-cloudregions have a zero velocity.

When apparatus 700 is configured to implement one or more of the stepsof methodology 300 the machine-readable medium may contain a programconfigured to (a) obtain sky images from each of at least one first skycamera system and at least one second sky camera system, wherein thefirst sky camera system and the second sky camera system each includes(i) an objective lens having a field of view of greater than about 170degrees, (ii) a spatial light modulator at an image plane of theobjective lens, wherein the spatial light modulator is configured toattenuate light from objects in an image captured by the objective lens,(iii) a semiconductor image sensor, and (iv) one or more relay lensconfigured to project the image from the spatial light modulator to thesemiconductor image sensor, and wherein the first sky camera system andthe second sky camera system are located at a distance of from about 100meters to about 1,000 meters from one another; (b) measure a position ofclouds in the images based on x,y,z axes coordinates of each of thefirst sky camera system and the second sky camera system; (c) determinean x′,y′,z′ orientation of each of the first sky camera system and thesecond sky camera system with respect to zenith and north as references;(d) determine an orientation of the clouds with respect to the zenithand north using the position of the clouds in the images measured instep (b) and the orientation of each of the first sky camera system andthe second sky camera system with respect to the zenith and northdetermined in step (c); and (e) calculate a position and a verticalheight of the clouds in the sky using triangulation based on theorientation of the clouds with respect to the zenith and north from step(d) and three-dimensional coordinates of the first sky camera system andthe second sky camera system. The machine-readable medium may be arecordable medium (e.g., floppy disks, hard drive, optical disks such asremovable media 750, or memory cards) or may be a transmission medium(e.g., a network comprising fiber-optics, the world-wide web, cables, ora wireless channel using time-division multiple access, code-divisionmultiple access, or other radio-frequency channel). Any medium known ordeveloped that can store information suitable for use with a computersystem may be used.

Processor device 720 can be configured to implement the methods, steps,and functions disclosed herein. The memory 730 could be distributed orlocal and the processor device 720 could be distributed or singular. Thememory 730 could be implemented as an electrical, magnetic or opticalmemory, or any combination of these or other types of storage devices.Moreover, the term “memory” should be construed broadly enough toencompass any information able to be read from, or written to, anaddress in the addressable space accessed by processor device 720. Withthis definition, information on a network, accessible through networkinterface 725, is still within memory 730 because the processor device720 can retrieve the information from the network. It should be notedthat each distributed processor that makes up processor device 720generally contains its own addressable memory space. It should also benoted that some or all of computer system 710 can be incorporated intoan application-specific or general-use integrated circuit.

Optional display 740 is any type of display suitable for interactingwith a human user of apparatus 700. Generally, display 740 is a computermonitor or other similar display.

As described above, multiple image sensors may be employed in thepresent sky camera system(s). To do so, a beam splitter may be needed.FIG. 8 is a schematic diagram illustrating how a dichromatic beamsplitter may be integrated into the present sky camera system. Themultiple image sensors are labeled Image Sensor 1 and Image Sensor 2. Inthis example, the dichromatic beam-splitter is inserted in the skycamera system before the relay lens. Thus, as highlighted above,multiple relay lenses (labeled Relay Lens 1 and Relay Lens 2) are usedfor coupling the image plane of the objective lens to the correspondingimage sensors.

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope of the invention.

What is claimed is:
 1. A sky camera system, comprising: an objectivelens having a field of view of greater than or equal to 170 degrees; aspatial light modulator at an image plane of the objective lens, whereinthe spatial light modulator is programmable and is configured toattenuate light from objects including direct sun in images captured bythe objective lens such that the sky camera system is configured toobtain sky images including an image of the sun; at least oneanti-blooming semiconductor image sensor behind the spatial lightmodulator; and one or more relay lenses, between the spatial lightmodulator and the anti-blooming semiconductor image sensor, configuredto project the images from the spatial light modulator to theanti-blooming semiconductor image sensor, wherein the spatial lightmodulator is configured to attenuate the image of the sun such that theimage of the sun can fit into a dynamic range of the anti-bloomingsemiconductor image sensor, and wherein a response of the sky camerasystem, prior to field operation, has been calibrated indoors using acalibration light source of known radiation intensity with the spatiallight modulator set to transparent, and wherein the sky camera system isfree of a mechanical sun blocker and thus can obtain a complete skyimage.
 2. The sky camera system of claim 1, wherein the objective lenscomprises a fish eye lens.
 3. The sky camera system of claim 1, whereinthe spatial light modulator comprises a liquid crystal modulator, aquantum well modulator, or a micro-blinds based modulator.
 4. The skycamera system of claim 1, wherein the anti-blooming semiconductor imagesensor comprises a charge-coupled device (CCD) sensor.
 5. The sky camerasystem of claim 1, wherein the anti-blooming semiconductor image sensorcomprises a complementary metal-oxide semiconductor (CMOS) sensor. 6.The sky camera system of claim 1, further comprising: a spectralfiltering component between the objective lens and the anti-bloomingsemiconductor image sensor configured to switch between desiredwavelength windows, wherein the spectral filtering component comprisesmechanically-switched filter wheels.
 7. The sky camera system of claim1, wherein the anti-blooming semiconductor image sensor comprises asensor covering from about 400 nm to about 1,100 nm spectral region. 8.The sky camera system of claim 1, wherein the sky camera systemcomprises multiple anti-blooming semiconductor image sensors andmultiple relay lenses, and wherein the sky camera system furthercomprises: a dichromatic filtering component between the objective lensand the multiple anti-blooming semiconductor image sensors configured tocapture images from multiple desired wavelength windows via the multiplerelay lenses each of which couples the image plane of the objective lensto a corresponding one of the multiple anti-blooming semiconductor imagesensors.
 9. The sky camera system of claim 8, wherein the dichromaticfiltering component comprises electrochromatic glass, liquid crystal, oracousto-optical grating.
 10. A total sky imaging system, comprising: atleast one first sky camera system and at least one second sky camerasystem, wherein the first sky camera system and the second sky camerasystem each includes (i) an objective lens having a field of view ofgreater than or equal to 170 degrees, (ii) a spatial light modulator atan image plane of the objective lens, wherein the spatial lightmodulator is programmable and is configured to attenuate light fromobjects including direct sun in images captured by the objective lenssuch that the first sky camera system and the second sky camera systemare each configured to obtain sky images including an image of the sun,(iii) at least one anti-blooming semiconductor image sensor behind thespatial light modulator, and (iv) one or more relay lenses, between thespatial light modulator and the anti-blooming semiconductor imagesensor, configured to project the images from the spatial lightmodulator to the anti-blooming semiconductor image sensor, wherein thespatial light modulator is configured to attenuate the image of the sunsuch that the image of the sun can fit into a dynamic range of theanti-blooming semiconductor image sensor, and wherein a response of thefirst sky camera system and the second sky camera system, prior to fieldoperation, has been calibrated indoors using a calibration light sourceof known radiation intensity with the spatial light modulator set totransparent, wherein the first sky camera system and the second skycamera system are each free of mechanical sun blockers and thus canobtain complete sky images, and wherein the first sky camera system andthe second sky camera system are located at a distance of from about 100meters to about 1,000 meters from one another.
 11. The total sky imagingsystem of claim 10, wherein the first sky camera system and the secondsky camera system each further includes (v) a spectral filteringcomponent between the objective lens and the anti-blooming semiconductorimage sensor configured to switch between desired wavelength windows.12. The total sky imaging system of claim 10, wherein the objective lenscomprises a fish eye lens.
 13. The total sky imaging system of claim 10,wherein the spatial light modulator comprises a liquid crystalmodulator, a quantum well modulator, or a micro-blinds based modulator.14. The total sky imaging system of claim 10, wherein the anti-bloomingsemiconductor image sensor comprises a CCD sensor.
 15. The total skyimaging system of claim 10, wherein the anti-blooming semiconductorimage sensor comprises a CMOS sensor.
 16. The total sky imaging systemof claim 10, wherein the anti-blooming semiconductor image sensorcomprises a sensor covering from about 400 nm to about 1,100 nm spectralregion.
 17. The total sky imaging system of claim 10, wherein the firstsky camera system and the second sky camera system each comprisesmultiple anti-blooming semiconductor image sensors and multiple relaylenses, and wherein the first sky camera system and the second skycamera system each further comprises: a dichromatic filtering componentbetween the objective lens and the multiple anti-blooming semiconductorimage sensors configured to capture images from multiple desiredwavelength windows via the multiple relay lenses each of which couplesthe image plane of the objective lens to a corresponding one of themultiple anti-blooming semiconductor image sensors.
 18. The total skyimaging system of claim 17, wherein the dichromatic filtering componentcomprises electrochromatic glass, liquid crystal, or acousto-opticalgrating.
 19. The sky camera system of claim 1, wherein the calibrationlight source of know radiation intensity is a calibration lamp on anintegration sphere.